We’ve all had days when we got out of the wrong side of the bed and the world looked greyer than usual. This daily variation in mood is a potential problem for psychologists who want to use tests to compare people’s mental ability – competing job candidates, for example. Mood, like tiredness and motivation, could be another factor that leads some people to perform below par, by their own standards, thus distorting the test results.

Indeed, there’s some evidence that being in a bad mood is distracting because it takes mental effort to deal with unpleasant emotions. Being in a good mood, by contrast, is thought to be energising. However, a new study in the journal Intelligence looked at how people’s mood and mental performance varied over five consecutive days and it actually found no link between the two.

Sophie von Stumm recruited 98 participants, mostly students (74 women; average age 24), to complete five different versions of the same three mental tests on five consecutive days, Monday to Friday. Seventy-seven participants turned up for all the sessions. One test concerned short-term memory (remembering lists of numbers or letters); another was a test of processing speed (comparing as quickly as possible whether strings of letters and numbers were identical); and the final test tapped working memory (involving mental arithmetic).

Each day, before beginning the mental tests, the participants completed a comprehensive measure of their current mood, using a sliding scale to indicate how much they were feeling 10 different positive emotions and 10 different negative emotions. Participants could arrive any time each day between 9 to 6 to complete the tests.

Von Stumm says she found “considerable” variability in the participants’ mental performance and their mood from one day to the next, with mood varying more than cognitive performance. But crucially, there was no coupling between the two. That is, daily changes in how well participants performed on the mental tests was not tied to daily fluctuations in their mood.

This result shouldn’t be taken to mean that serious emotional distress is not harmful to mental performance, but the results do suggest that mundane fluctuations in our mood are unlikely to affect our mental performance. So if you’re in a grump today, take heart – at least it’s unlikely to slow you down mentally.

Taking a more sceptical view, note the relatively small sample size and the fact the study only looked at fluctuations over five days. It’s possible the findings might differ over narrower (multiple tests in one day) or longer timescales.

Incidentally, there was a significant link between participants’ average positive mood across the study and their test performance – that is, participants who were generally in a better mood across the five days tended to perform better than less happy participants on the mental tests. “Put bluntly,” von Stumm said, “this suggests that people who have a general tendency to be more enthusiastic and alert have faster brains, but additional research will be needed to substantiate this observation.”